TOPIC 5: MATTER ~ CHEMISTRY FORM 1

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Which of the substances under investigation can compress into a smaller volume?

You should have found that a solid (sand) and a liquid (water) cannot be compressed but a gas (air) is easily compressed.

The three states of matter differ in their physical properties. These differences in properties are summarized in table bellow

Melting is a change from solid to liquid state. When solids are heated, their constituent particles (atoms, molecules or ions) get energy and vibrate more violently. Vibrations of these particles overcome (exceed) their binding forces. The particles become mobile. The crystalline structure of solid is destroyed. A liquid state is reached and the particles are free to move. The temperature at which this happens is called melting point of the solid.

The melting point of a solid tells us something about the strength of forces holding its constituent particles together. Substances with high melting points have strong forces between their particles. Those with low melting points have weak forces between their particles.

Freezing is a change from liquid to solid state. Freezing is the opposite of melting. The process is reversed at the same temperature if a liquid is cooled. The temperature at which a substance turns to a solid is called freezing point. The melting point and freezing point of any given substance are both the same. For example, the melting and freezing of pure water takes place at 0°C. Melting is not affected by any changes in atmospheric pressure.

Boiling is a change from liquid to vapour state at a particular temperature. Evaporation is the change from liquid to vapour state at any given temperature. If a liquid is exposed to open air, it evaporates. Splashes of water evaporate at room temperature. After rain, small pools of water dry up. When a liquid changes into a gas at any temperature, the process is called evaporation. Evaporation takes places from the surface of the liquid. The larger the surface area, the faster the liquid evaporates. The warmer the liquid is, the faster it evaporates. Thus, surface area and temperature affects the rate of evaporation of a liquid.

When a liquid is heated, its molecules get more energy and move faster. They knock into each other violently and bounce further apart. As the heating goes on, its molecules vibrate even faster. Bubbles of gas (due to air dissolved in water) appear inside the liquid. The whole process is called boiling. The temperature at which a liquid boils is called boiling point.

The molecules at the surface of the liquid gain enough energy to overcome the forces holding them together. They break away from the liquid and from a gas (vapour). As more of the liquid molecules escape to form a gas, a liquid is said to evaporate. This occurs at the boiling point of a liquid.

The temperature at which a liquid boils explains how strong the forces holding its particles (molecules) together are. Liquids with high boiling points have strong forces of attraction between their molecules than those liquids with low boiling points.

The boiling point of a liquid can change if the surrounding pressure changes. If the surrounding pressure falls, the boiling point also falls. The boiling point of water at standard pressure (760 mmHg) is 100°C. On a high mountain, where pressure is low, it is lower than 100°C. If the surrounding pressure is increased, the boiling point rises. The same behaviour is experienced by a gas when the pressure is either increased or decreased.

From the above explanation, obvious differences between evaporation and boiling can be detected. See table bellow

Therefore, the two terms can be defined as follows: Evaporation is a change in state of a substance from liquid to gas (vapour) state at any temperature.

Boiling is a change in state of a substance from liquid to gas at a particular temperature and pressure.

The reverse of evaporation is condensation. This is brought about by cooling. When a gas is cooled down, its particles lose energy. They move more and more slowly. When they knock into each other, they do not have enough energy to bounce away again. They stay close together and a liquid forms. This process is called condensation. When the liquid is cooled further, the movement of the particles slows down even more. Eventually, they stop moving and a solid forms. This is called solidification.

Condensation can be defined as a change in state of a substance from gas (vapour) to liquid. Solidification is a change from liquid to solid state of a substance. Solidification is the same as freezing.

A few solids do not melt when they are heated. Instead, they change directly from the solid to gaseous state without passing through the liquid state. This change in state is called sublimation. When a solid changes directly into gas, it is said to sublime. Iodine, solid carbon dioxide (“dry ice”) and ammonium chloride are examples of solids that sublime. Like melting, sublimation also occurs at one particular temperature for each pure solid.

Different mixtures can be separated through such processes as distillation, sublimation, evaporation and condensation. Let us have a look at an example of distillation. This process involves boiling, evaporation and condensation. Distillation as a process can be applied in separation of a mixture (solution) of two or more substances. A mixture of two or more substances with different boiling points e.g. water and alcohol can be separated by this means. In such a case, a container with the mixed-up liquids is heated. The liquid with a low boiling point evaporates and condenses first, leaving the one with a high boiling point in the container. The distillate (liquid with low boiling point) is collected, cooled down and transferred into another container.

Industrially, the process of distillation is applied in the production of pure substances such as beer and other alcoholic drinks such as wine, vodka, konyagi, etc. The manufacturing process involves boiling, evaporating and condensation.

Crude oil contains organic liquid components, each with a different boiling point. In the refinery, the components with lower boiling points evaporate first and get separated out, leaving those with higher boiling points behind. In this way, we get various types of oil components (fractions) such as petrol, diesel, kerosene, lubricating oil, etc.

When you suspend your clothing on a cloth line to dry, the moisture in it is lost through evaporation. Likewise, farmers in the village often spread crops on the ground to dry. They do this in order to reduce moisture content and hence prevent decaying. The moisture contained in crops leave by evaporation. Therefore, you can notice how evaporation, as a change in state, is important in everyday lives.

You all like to drink cold water or beverages especially during hot weather. You can use a refrigerator to cool down drinking water or beverages directly. Alternatively, you can freeze water into ice and then use the resulting ice for cooling the beverage. Ice blocks are also saleable. Moreover, one can earn some money if she freezes water into ice blocks and then sells them to beverage vendors. Perishable products such as fish, meat, milk, etc are often packed in ice blocks to prevent them from going bad. Ice, as we studied early, is formed when water freezes (a change in state from liquid to solid).

You all like to drink cold water or beverages especially during hot weather. You can use a refrigerator to cool down drinking water or beverages directly. Alternatively, you can freeze water into ice and then use the resulting ice for cooling the beverage. Ice blocks are also saleable. Moreover, one can earn some money if she freezes water into ice blocks and then sells them to beverage vendors. Perishable products such as fish, meat, milk, etc are often packed in ice blocks to prevent them from going bad. Ice, as we studied early, is formed when water freezes (a change in state from liquid to solid).

In metallurgical industries, need may arise to mix two or more metals (alloys) together. This is only possible, where two or more metals are first melted at high temperatures into liquids. Then the resulting liquid metals are mixed in appropriate proportions. This is followed by cooling down the mixture to a solid alloy. Normally alloys have better qualities than individual metals.

The presence of impurity may lower or raise the boiling point of the substance. A pure substance melts and boils at definite temperatures (see table 5.4). The values for the melting point and boiling point are precise and predictable. This means that we can use them to test the purity of a sample. They can also be used to check the identity of unknown substance.

Sea water is impure. It freezes at a temperature well below the freezing point of pure water (0°C) and boils at a temperature above the boiling point of pure water (100°C). Other substances behave in a similar manner. So, boiling as a change in state can be used to test for the purity of a substance.

In addition, the impurity also reduces the exactness of the melting or boiling point. An impure substance melts or boils over a range of temperature, not at a particular point.

Perhaps the most important of all, as far as change in state is concerned, is the formation of rain. Rained is mainly formed through the process of evaporation and condensation. Water vapour, evaporating mostly from water bodies (oceans, seas, lakes, rivers, ponds, etc), land and plants rises up to the sky. As it rises, it cools down and condenses into tiny droplets

On further cooling as they rise up, these droplets form bigger water drops. Owing to gravitational force, these drops fall down as rainfall. Every one of you knows how important rain is to our life. Therefore, you have noticed how evaporation and condensation, as changes in state, contribute to rain formation.

We already know that matter is composed of atoms, ions or molecules. We have not yet considered the reason why the same substance, say water, can exist in more than one form, for example as solid ice, liquid water, and gaseous steam. But does matter behave like that?

The kinetic theory of matter has been used to explain the way in which the arrangement of the particles of a substance can determine the properties of that substance, and particularly the state in which it is likely to be found under a given set of conditions. The idea is that all matter is made up of tiny moving particles. The main points of the theory are as follows:

In the solid state, the particles are so closely packed (see figure bellow. The particles are held together by strong forces of attraction that act like a chemical glue. Free movement of particles cannot take place. They cannot move around freely in this arrangement. Instead, they vibrate about a fixed position. They are arranged in a fixed pattern which form a cluster of vibrating masses. This makes a solid to have a fixed shape, which cannot be changed except by applying strong external forces.

The particles of a liquid are also closely packed but the forces of attraction between them are weaker than of a solid. These forces of attraction tend to bind them together. The particles have more kinetic energy and they can move around each other. The binding forces are strong when particles come close to one another. It is thought that the particles of a liquid are fairly randomly arranged but consist of “clusters” closely packed together. This property makes a liquid to have a definite volume. However, since the particles are fairly free to move a liquid does not have any characteristic shape (see figure 5.5(b). Thus, a liquid will always take the shape of its container.

The gaseous state is one in which the particles are moving independently of each other in all directions and at great speeds. The particles of a gas are relatively far apart. They exert no force of attraction on each other. They have more energy than the particles of solids and liquids. They move rapidly and randomly, colliding with each other and with the walls of the container. A typical speed for a molecule of hydrogen in air at ordinary temperature and pressure has been found to be approximately 500 ms^-1

It has been estimated that a nitrogen molecule makes collisions each second. Thus, a gas will rapidly spread out to fill any container in which it is placed. A gas cannot have any shape of its own.

Substances may undergo changes in their physical properties e.g. changes in colour, shape (or form), state, density, structure and texture, etc. If you take a stone and break it down into small particles, you will have only changed its form, but it will remain as a stone. Likewise, melting ice to water or freezing water to ice does not change it, but it is still water. The same case happens when you dissolve salt in water to get a solution of salt in water. You can still get back the original salt by evaporation, except that the crystals of the salt obtained will not look exactly the same as those of the original salt.

These changes of state are examples of physical changes. Physical changes such as melting and boiling do not result in new substances being formed

For example, ice and water still contain the same particles whether in solid (ice) liquid (water) or gaseous (vapour) state.

In the explanation above, we find that in a physical change it is only the physical form, and not the actual nature, of a substance that changes. The changes are brought about by a mere addition or removal of heat, as in the case with water or ice. Such a change is called a physical change. It can be distinguished by the following characteristics:

Some changes that materials undergo are permanent. Such changes usually involve changes in chemical properties of a substance. For example, when you burn a piece of wood in fire, you get ash. The properties of wood and ash are very different. There is no way you can change ash back to wood. It is practically impossible. A permanent change in chemical properties of a substance is called a chemical change. In a chemical change, a substance losses all its physical and chemical properties.

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A chemical symbol is the way of representing an element using initial letter(s). There are many different elements as you have seen above. Every element has a name and a symbol to represent it. Some symbols are just a single capital letter, such as H. Others have two letters, the first of which is always a capital, such as Mg.

Includes

All symbols are recognized and are used by all scientists all over the world. Some examples of elements and their symbols are given in the table below

t is easy to remember that the symbol for aluminium is Al, and for carbon is C. But some symbols are harder to remember because they are taken from Latin names. For example, potassium has the symbol, K from its Latin name Kalium. Sodium has the symbol, Na from its Latin name Natrium. See the complete list in the following table.

The reason for assigning some elements with Latin names was to avoid confusion among scientists when representing different elements. For example, the symbol for silicon is Si. It could be impossible to represent silicon by the symbol S and at the same time represent the element sulphur by the very symbol, S. Similarly, potassium could not be represented by the symbol P that was assigned to phosphorus. So in order to avoid such confusion, scientists decided to use Latin names to represent some elements. In so doing, the anticipated and unnecessary contradiction among scientists from different parts of the world was avoided.

Symbols are particularly useful when more than one atom is present in a substance. For example, hydrogen gas consists of pairs of hydrogen atoms joined together. So hydrogen gas is shown as H₂. When more than one atom is joined together like this, we call the substance formed a molecule. Atoms making up gases such as hydrogen, oxygen, nitrogen, etc, always exist as molecules. Sulphur exists as a hexagonal ring of eight atoms. Phosphorus exists as a tetrahedron of four atoms. Table bellow shows some elements that exist as molecules.

Elements can be classified as either metals or non-metals. Metals and non-metals have different physical and chemical properties. This is the criterion used for classification of these elements into metals on one hand and non-metals on the other hand. Table bellow summarizes the physical and chemical properties of some common elements.

There are 94 naturally occurring elements. Some of them are very rare. Francium, for instance, has never been seen. The radioactive metals neptunium and plutonium, which we make artificially in quite large amounts, only occur in very small (trace) quantities naturally. Most of the elements can be classified as metals. The rest are non-metals. To understand these elements better, refer to the Periodic Table of Elements at the back of this book. Classification of elements into metals and non-metals is based on differences between their physical and chemical properties. Differences between metals and non-metals are shown in table bellow.

Mixtures can be classified as solutions, suspensions or emulsions. This classification is based on whether the mixed substances dissolve completely or not. It also depends on the nature of the mixtures that result upon mixing. Let us look at each category in detail.

A solution is a uniform mixture of two or more substances. Such mixtures may be a solid in a liquid, a liquid in a liquid, a liquid in a gas and, very rarely, a gas in a gas. (See table bellow). We most often think of a solution as being made of a solid dissolved in a liquid. For example, solutions of sugar or salt in water are quite common. A solid that dissolves in a liquid is called a solute while the liquid in which that solid dissolves is called a solvent. For example, sugar and salt are solutes and water is a solvent.

However, other substances that are not normally solids can be found dissolved in a liquid. For example, the gases, carbon dioxide and oxygen, dissolved in water are important for life to continue in oceans, seas, lakes, rivers, etc.

Less obvious perhaps, but quite common, are solutions of one liquid in another. Alcohol mixes (dissolves) completely with water. Beer, wine and whisky do not separate into layers of alcohol and water (even when the alcohol content is quite high). Alcohol and water are completely miscible, that, is they make a solution.

Solutions of gases in gases are very uncommon. Technically, air could be described as a solution of several gases in nitrogen, though this could be unusual everyday use of the term. However, it is interesting to note that different gases always mix completely with each other.

A suspension is a cloudy mixture of solid particles suspended in a liquid. A solid is said to be in suspension in a liquid when small particles of it are contained in a liquid, but are not dissolved in it. If the mixture is left undisturbed, the solid particles will slowly settle to the bottom of the containing vessel, leaving the pure liquid above them.

Muddy water is a typical suspension. The mud would settle after a time if left undisturbed leaving brown residue on the bottom of the containing vessel and clear water above. The particles of mud would be retained by filtering whilst the water (and any solids in solution) would pass through.

If you mix flour or chalk dust in water, it forms a suspension. Their particles are simply dispersed (spread) throughout the water and would eventually settle down to the bottom of the vessel if left undisturbed for sometime.

An emulsion is a cloudy mixture of tiny droplets of one liquid suspended in another liquid. Sometimes two immiscible liquids will not separate out into two layers when mixed together. One of the liquid may form droplets and spread throughout the other to form an emulsion. Cooking oil and water do not mix but they will form an emulsion when they are mixed and shaken. Droplets of oil will spread throughout the water. Unlike pure liquids, emulsions are cloudy (opaque). So you cannot see through them. The emulsion will not settle like a suspension. Which other liquids you know can form suspensions?

Mixtures can be formed from different substances in two major ways.

The first type constitutes homogenous mixtures, where the substances are totally mixed together uniformly. Examples include solutions of salts and sugars in water.

The second type constitutes heterogeneous mixtures, where the substances remain separate and one substance is spread throughout the other as small particles, droplets, or bubbles. All emulsions and suspensions fall under this category. Examples include suspensions of insoluble solids or oil droplets in water.